Plasma and whole blood histamine levels and white cell counts have been monitored over a 6-week period of storage in standard citrate-phosphate-dextrose supplemented with adenine (CPDA) blood packs, add-back blood packs, and also CPDA and add-back packs which were subjected to a microfiltration process at the time of collection. Our results indicate that the plasma histamine levels rises and the white blood count falls progressively with time, the former being most marked after the 3rd week of blood storage. At 6 weeks, there was no significant difference between the plasma and whole blood histamine level, suggesting that the entire complement of histamine in blood was now in plasma. Microfiltration substantially reduced the white cell count in the packs, and this was reflected in a greatly reduced whole blood and (subsequently) plasma histamine concentration.